Thermal remediation of contaminated soils and solid waste ("soil" and "soil") is a well known and effective technology. However, treatment cost is a major problem. Most thermal treatment methods require excavation and decontamination above ground, either on-site or at a remote treatment location, and then replacement of the treated soil.
Excavation and disposal of contaminated soil in landfills is also expensive and is becoming more and more restricted by law. It is also risky in terms of future liability. In some contaminated soil cases, such as contamination caused by tank leaks in urban or industrial areas, excavation is very expensive and sometimes even impossible without causing damage to surrounding structures. In situ treatment methods have distinct advantages over these methods in that they avoid extensive excavation costs and increase safety during treatment.
Possible in situ treatment methods involve heating the soil with electromagnetic energy radiated from an antenna placed in a wellbore in the ground. When a single wellbore is heated with electromagnetic (EM) energy, radiated from an antenna, the EM field attenuation, as a function of the radius, is dominated by two factors: (1) cylindrical attenuation of the field power strength (by a factor of 1/x for a long cylinder), and (2) exponential decay due to soil penetration depth, defined as the distance where power is attenuated to 0.37 of its initial surface value.
The penetration depth of EM energy is inversely related to the frequency, the apparent AC conductivity, and the loss tangent of the material. At the beginning of the heating process, this penetration depth is small due to the moisture of the soil. As the heating process advances, the moisture is evaporated in growing radial layers. The maximum penetration depth that is reached is that achieved when the soil is dry.
EM heating creates a highly nonuniform temperature profile along the well radius which is proportional to 1/r e.sup.-(r-r0)/.beta.0 (where r.sub.0 is the physical well radius, r is the radial distance from the well center, and .beta..sub.0 is the EM penetration depth). The result of the excessive overheating near the well and underheating away from the well is poor application efficiency of the electromagnetic energy (0.25-0.37).
Another problem arising from the nonuniform heating is that for large ratios of the wellbore heating radius to the wellbore physical radius, the temperature at the wellbore is very high, imposing severe limitations on well materials and causing overheating problems.
Uniform heating is also not possible in a single wellbore even when both low frequencies (large penetration depth) and thermal conductivity are used. The reason for this is that the formation is continuously conducting the heat outward. If heat transfer is used to get a more uniform heating for certain distances, the same heat conduction property of the soil will dissipate the heat to the surrounding zone.
A vacuum extraction (VE) method can also be used to remove contamination from the ground. This method, however, is only effective for a narrower range of contaminants than thermal treatment methods are, and it entails very long treatment times (up to years) which are also difficult to predict. In this method (VE), only contaminants with relatively high vapor pressure are removed. Low vapor pressure fractions of these materials are left behind. Materials such as diesel fuel, for example, constitute of a variety of compounds with high and low vapor pressure. Vacuum extraction therefore, will selectively remove the high vapor pressure. Vacuum extraction therefore, will selectively remove the high vapor pressure fraction of contamination and leave behind high molecular weight low vapor pressure materials.
U.S. Pat. No. 4,886,118 to Van Meurs et al. describes a method of producing oil from substantially impermeable subterranean oil shale by use of heaters (electrical or others) placed in wells which conduct heat in the oil shale. The use of heat conduction alone, to transfer the heat from the wellbore physical radius outside limits the power and heating rate for a given maximum allowable wellbore temperature, resulting in very long heating times. In addition, separation of heating and production wells increases costs by increasing the total number of wells by a factor of 3.
U.S. Pat. No. 4,376,598 to Brouns et al. discloses in situ vitrification of soils by conduction heating at very high temperatures (1500.degree. C.) in order to immobilize radioactive contaminants. The vitrification process is an expensive method, requiring very high energy.
U.S. Pat. No. 4,670,634 to Bridges et al. discloses an in situ radio frequency (RF) heating method for decontamination of soils. Contaminated soil is subjected to RF energy via an array of electrodes placed above the ground. Soil heating results from the fringe field of the electrodes. As a result of the fact that the electrodes are placed above ground, heating depth is very limited (about 3 feet) and the temperature profile is strongly nonuniform.
U.S. Pat. No. Re. 30,738 to Bridges discloses an apparatus and method for RF in situ heating to produce petroleum products. A plurality of conductors are inserted in the formation and bound particular volumes of the formation. This is equivalent to a "triplate" parallel capacitor, with relatively uniform field, except for the edges. One disadvantage of this system is that it can only heat an area of rectangular shape. Since volumes of contamination are rarely of rectangular shape, this system is inefficient because the heated region includes areas that are not contaminated. This system is particularly inefficient in medium and small sites that cannot be economically subdivided into smaller modules. It is also inefficient for decontamination of soils due to tank leaks or nonuniform spreading leaks, or point leaks at hazardous waste disposal sites, or mixed chemical and radioactive waste disposal sites, landfills, or underground tanks containing a mixture of radioactive and chemical waste and the like which might have irregular, elongate contamination regions. Other disadvantages of this method and other in situ RF heating methods compared to shorter microwave methods are:
(1) Metal objects such as pipes, metal drums and other metal objects buried in the contaminated ground or in landfills, or underground utilities such as buried electrical or telephone lines, water or sewer lines, will alter the desired electric field pattern resulting in nonuniform heating, and PA1 (2) Bound wave RF structures are very sensitive to the vast changes of the soil electrical properties, especially during moisture evaporation, which complicates impedance matching of the generator to the soil.
U.S. Pat. No. 4,817,711 to Jeamby discloses a system for recovery of pertroleum from a well by microwave heating. This system provides nonuniform heating and very long heating times.
U.S. Pat. No. 4,590,348 to Lahti et al. discloses a system for thawing frozen ground by microwave heaters. The heating is local heating with a relatively small heating range limited by the immediate effect of the EM soil penetration depth.